AMBIO (2011) 40:17–31 DOI 10.1007/s13280-011-0212-y The Changing Face of Arctic Snow Cover: A Synthesis of Observed and Projected Changes Terry V. Callaghan, Margareta Johansson, Ross D. Brown, Pavel Ya. Groisman, Niklas Labba, Vladimir Radionov, Roger G. Barry, Olga N. Bulygina, Richard L. H. Essery, D. M. Frolov, Vladimir N. Golubev, Thomas C. Grenfell, Marina N. Petrushina, Vyacheslav N. Razuvaev, David A. Robinson, Peter Romanov, Drew Shindell, Andrey B. Shmakin, Sergey A. Sokratov, Stephen Warren, Daquing Yang Abstract Analysis of in situ and satellite data shows Keywords Snow depth Á Snow water equivalent Á evidence of different regional snow cover responses to the Snow cover duration Á Snow cover extent widespread warming and increasing winter precipitation that has characterized the Arctic climate for the past 40–50 years. The largest and most rapid decreases in snow INTRODUCTION water equivalent (SWE) and snow cover duration (SCD) are observed over maritime regions of the Arctic with the Frozen precipitation accumulating on a surface creates a highest precipitation amounts. There is also evidence of snow cover. Snow is an important and dominant feature marked differences in the response of snow cover between of Arctic terrestrial landscapes with cover present for the North American and Eurasian sectors of the Arctic, 8–10 months of the year. Its extent, dynamics, and prop- with the North American sector exhibiting decreases in erties (e.g., depth, density, water equivalent, grain size, and snow cover and snow depth over the entire period of changes in structure throughout its vertical profile) affect available in situ observations from around 1950, while climate (e.g., ground thermal regime), human activities widespread decreases in snow cover are not apparent over (e.g., transportation, resource extraction, water supply, use Eurasia until after around 1980. However, snow depths are of land, and ecosystem services), as well as infrastructure, increasing in many regions of Eurasia. Warming and more hydrological processes, permafrost, extreme events frequent winter thaws are contributing to changes in snow (including hazards such as avalanches and floods), biodi- pack structure with important implications for land use and versity, and ecosystem processes. Snow is therefore a provision of ecosystem services. Projected changes in snow significant component in the socio-economics of Arctic cover from Global Climate Models for the 2050 period societies. The important physical properties that exert an indicate increases in maximum SWE of up to 15% over influence on climate or moderate its effects (Cohen and much of the Arctic, with the largest increases (15–30%) Rind 1991) include high short-wave albedo, high thermal over the Siberian sector. In contrast, SCD is projected to emissivity, low heat conductivity, large latent heat of decrease by about 10–20% over much of the Arctic, with fusion, and low surface roughness while it stores and rap- the smallest decreases over Siberia (\10%) and the largest idly releases water in the melt season. The combination of decreases over Alaska and northern Scandinavia (30–40%) high albedo and low thermal conductivity promotes low by 2050. These projected changes will have far-reaching surface temperatures and low-level temperature inversions. consequences for the climate system, human activities, The low thermal conductivity of snow allows it to insulate hydrology, and ecology. the surface from large energy losses in winter, and this has major implications for the development of seasonally frozen ground and permafrost. The characteristics of Arctic snow cover are the result of a complex interplay of atmospheric and surface processes Electronic supplementary material The online version of this article (doi:10.1007/s13280-011-0212-y) contains supplementary that determine not only the quantity of water stored as material, which is available to authorized users. snow, but also snowpack condition (e.g., grain size, Ó Royal Swedish Academy of Sciences 2012 www.kva.se/en 123 18 AMBIO (2011) 40:17–31 density, and ice layers). The amount of snow accumulating Because the Arctic’s snow cover is strongly related to on a surface is influenced by precipitation amount, type, temperature and moisture as described above, past (Walsh and timing; blowing snow transport and sublimation; and et al. 2011a, b [this issue]) and projected changes in the vegetation interception. However, the character and evo- Arctic’s temperature and precipitation are likely to result in lution of high-latitude snowpack has the additional com- changes in the characteristics of the Arctic’s snow cover plexity of being particularly strongly dependent on blowing with far-reaching impacts on the climate system (Callaghan snow processes with the distribution and physical proper- et al. 2011a, b [this issue]), human activities, as well as ties of snow on the ground closely linked to local-scale infrastructure, hydrological processes, permafrost, extreme variability in terrain and vegetation (King et al. 2008). The events (including hazards such as avalanches and floods), key large-scale physiographic and climatic factors influ- biodiversity, and ecosystem processes (AMAP 2011; Cal- encing the regional distribution of Arctic snow cover (see laghan et al. 2011b [this issue]). This article assesses cur- On-line supplementary material Fig. A) are elevation, rent and projected changes in the Arctic’s snow cover. It is amount of vegetation cover, spatial distribution of freezing part of a larger assessment of the Arctic’s entire cryosphere temperatures, and location of the main cyclone tracks (AMAP 2011; Callaghan et al. 2011b [this issue]). bringing moisture into the Arctic. Air temperature and elevation exert the strongest influences on the distribution of snow cover duration (SCD) across the Arctic (Fig. Ae) CURRENT CHANGES IN SOLID PRECIPITATION with both continents exhibiting marked east-west increases in snow cover in response to the modification of winter air There is a wide range of regularly observed snow cover masses over the cold, dry continental interiors. Land areas information in the Arctic from in situ and satellite obser- in the zone of -20°C mean winter temperatures (see darker vations. The SCD on the ground is one of the best-observed blue area in Fig. Ac) experience snow cover for most of the variables in terms of resolution and longevity. Snow depth year. The spatial distribution of snow water equivalent and SWE are more difficult to monitor due to their high (SWE; the depth of liquid water that would result from spatial variability, large gaps in the in situ observing net- melting the snow) is more complex than SCD but is basi- works, and difficulties in monitoring from satellites. The cally driven by moisture availability over the snow season, indigenous peoples of the Arctic have a profound knowl- reflected in the cyclone frequency map (Fig. Ad). The edge of changing snow conditions of practical importance highest snow accumulations in the Arctic are located in the for survival, which has been passed from generation to coastal mountain regions and considerably more moisture generation, and the Sa´me observe snow stratigraphy that is penetrates into the western sector of the Eurasian Arctic important for reindeer access to vegetation (Riseth et al. than North America, where the coastal mountains block 2010). moisture entering from the Pacific Ocean. Regions with In the Atlantic, North European, and West Siberian winter temperatures closer to freezing, such as Scandinavia sectors, the climatic conditions are formed largely under and the Pacific coasts of Russia and Alaska, are also more the influence of heat and moisture advection from the likely to experience thaw and rain-on-snow events that North Atlantic area. Climate in the East Siberian and create ice layers in the snowpack. Chukchi sectors is significantly influenced by circumpolar The high winds, low temperatures and low snowfall conditions over the northern Pacific Ocean, as well as by amounts over the exposed tundra regions of the Arctic the center of action above Siberia (see Fig. 1 for definition produce a snow cover that is typically quite shallow, about of the sectors). The Alaskan sector is also influenced by 30–40 cm (except in drifts and gullies), with a wind-hard- circumpolar processes over the northern Pacific Ocean. In ened surface layer (‘‘wind slab’’) overlying a less dense the Canadian sector, the climatic conditions in winter are depth hoar (‘‘sugar snow’’) layer (Derksen et al. 2010). The governed both by anticyclonic circulation above north- average snow density remains close to 300 kg m-3 over western Canada and the Arctic Basin and by the frequent much of the snow season, but snow depth and properties can passage of Alberta lows and Atlantic east coast systems. exhibit strong local variation with many exposed areas, Analysis of trends in seasonal totals of precipitation drifts, dunes, and zastrugi (sharp irregular ridges on the from October to May (which correspond to the snowfall snow surface formed by wind erosion and deposition). In season with mean monthly temperatures below -2°C) at forested regions of the Arctic (taiga and boreal forest), snow climate stations located north of 60°N revealed an increase cover is more uniform and less dense (*200 kg m-3) as the in cold season precipitation between 1936 and 2009 in trees act as windbreaks and shade the snow from incoming almost all sectors of the Arctic (Table 1; see also Figs. B solar radiation in the spring (McKay and Gray 1981). In and C in the On-line supplementary material). contrast, north of the tree line, where wind action compacts The analysis is based on monthly total precipitation data the snow, snow density is higher. collected at the stations from the start of their operation up Ó Royal Swedish Academy of Sciences 2012 123 www.kva.se/en AMBIO (2011) 40:17–31 19 Fig. 1 Long term meteorological stations in the Arctic (red dots) and mean monthly location of North Pole drifting stations (blue dots)in different sectors of the Arctic.